Induction of mutations in DNA-repair deficient bacteria by a liver microsomal metabolite of aflatoxin B1

The Biosynthesis, Structure Diversity and Bioactivity of Sterigmatocystins and Aflatoxins: A Review

Sterigmatocystins and aflatoxins are a group of mycotoxins mainly isolated from fungi of the genera Aspergillus. Since the discovery of sterigmatocystins in 1954 and aflatoxins in 1961, many scholars have conducted a series of studies on their structural identification, synthesis and biological activities. Studies have shown that sterigmatocystins and aflatoxins have a wide range of biological activities such as antitumour, antibacterial, anti-inflammatory, antiplasmodial, etc. The sterigmatocystins and aflatoxins had been shown to be hepatotoxic and nephrotoxic in animals. This review attempts to give a comprehensive summary of progress on the chemical structural features, synthesis, and bioactivity of sterigmatocystins and aflatoxins reported from 1954 to April 2024. A total of 72 sterigmatocystins and 20 aflatoxins are presented in this review. This paper reviews the chemical diversity and potential activity and toxicity of sterigmatocystins and aflatoxins, enhances the understanding of sterigmatocystins and aflatoxins that adversely affect humans and animals, and provides ideas for their prevention, research and development.

Joint Analysis of Microbial and Immune Cell Abundance in Liver Cancer Tissue Using a Gene Expression Profile Deconvolution Algorithm Combined With Foreign Read Remapping

Recent transcriptomics and metagenomics studies showed that tissue-infiltrating immune cells and bacteria interact with cancer cells to shape oncogenesis. This interaction and its effects remain to be elucidated. However, it is technically difficult to co-quantify immune cells and bacteria in their respective microenvironments. To address this challenge, we herein report the development of a complete a bioinformatics pipeline, which accurately estimates the number of infiltrating immune cells using a novel Particle Swarming Optimized Support Vector Regression (PSO-SVR) algorithm, and the number of infiltrating bacterial using foreign read remapping and the GRAMMy algorithm. It also performs systematic differential abundance analyses between tumor-normal pairs. We applied the pipeline to a collection of paired liver cancer tumor and normal samples, and we identified bacteria and immune cell species that were significantly different between tissues in terms of health status. Our analysis showed that this dual model of microbial and immune cell abundance had a better differentiation (84%) between healthy and diseased tissue. Caldatribacterium sp., Acidaminococcaceae sp., Planctopirus sp., Desulfobulbaceae sp.,Nocardia farcinica as well as regulatory T cells (Tregs), resting mast cells, monocytes, M2 macrophases, neutrophils were identified as significantly different (Mann Whitney Test, FDR< 0.05). Our open-source software is freely available from GitHub at https://github.com/gutmicrobes/PSO-SVR.git.

An evaluation of the metabolic basis of aflatoxin B1 toxicity by using buffalo granulocytes and agranulocytes in vitro

This study was aimed at monitoring cytotoxic changes in buffalo leukocyte subpopulations exposed to aflatoxin B1 (AFB1), since no such information is available for this species. The effects of AFB1 on glutathione (GSH) S-transferase, Ca2+Mg2+ATPase and protein synthesis in leukocyte subpopulations, namely, mononuclear (MN) cells and polymorphonuclear (PMN) cells isolated from the blood of the domestic buffalo (Bos bubalis), were studied. The cells were separated by using Ficoll-Paque and incubated in the presence of AFB1. GSH S-transferase activity was found to increase in cells exposed to AFB1, but there was no difference in activity between MN and PMN cells. PMN cell ATPase activity increased after AFB1 treatment, whereas no such effect was observed in the MN cells, which showed higher basal levels of ATPase activity. In the presence of AFB1, all the cells showed significant decreases in 14C-leucine incorporation, but the MN cells showed higher 14C-leucine incorporation than the PMN cells. Nevertheless, both cell types were affected by AFB1 and participated in its detoxification. There was also an appreciable decrease in the release of myeloperoxidase by activated PMN cells in the presence of AFB1.

Contribution of the glutathione S-transferases to the mechanisms of resistance to aflatoxin B1

The harmful effects of Aflatoxin B1 (AFB1) are a consequence of it being metabolized to AFB1-8,9-epoxide, a compound that serves as an alkylating agent and mutagen. The toxicity of AFB1 towards different cells varies substantially; sensitivity can change significantly during development, can be modulated by treatment with xenobiotics and is decreased markedly in preneoplastic lesions as well as in tumors. Three types of resistance, namely intrinsic, inducible and acquired, can be identified. The potential resistance mechanisms include low capacity to form AFB1-8,9-epoxide, high detoxification activity, increase in AFB1 efflux from cells and high DNA repair capacity. Circumstantial evidence exists that amongst these mechanisms the glutathione S-transferases, through their ability to detoxify AFB1-8,9-epoxide, play a major role in determining the sensitivity of cells to AFB1.

Sequential monitoring of cytogenetic damage in rat lymphocytes following in vivo exposure to aflatoxin B1 and N-nitrosophenacetin

Rats were treated intraperitoneally with different concentrations of aflatoxin B1 (AFB1) or N-nitrosophenacetin (NP). Blood was sequentially drawn by venous puncture at 6, 24, 72, 120 h and 14 days after a single injection of AFB1 or NP. After AFB1 the frequency of SCEs and chromosome aberrations increased progressively and reached a maximum level after 24 h and then decreased with time. By 2 weeks post treatment, the SCE and chromosome aberration values were within the control range. A small but significant SCE induction was observed when rats were treated with NP, but no chromosome breakage was induced even at the highest dose (20 mg/kg). We suggest that the elimination of DNA damage by repair mechanisms and lymphocyte turnover is responsible for the reduction of SCEs and chromosome aberrations with time. This assay seems promising for sequential monitoring of cytogenetic damage in rat lymphocytes following in vivo exposure to genotoxicants.

Effect of antiserum against aflatoxin B1-bovine serum albumin complex on aflatoxin B1-induced lysogenesis

Escherichia coli K12 bacteria lysogenic for the lambda phage were used to study the effect of antiserum against aflatoxin B1-induced lysogenesis. The antiserum was obtained from rabbits immunized with water in oil emulsion of aflatoxin B1-bovine serum albumin complex (AFB1-BSA). A marked reduction in the degree of lysogenesis was observed when the antiserum was added to the reaction medium prior to microsomal enzyme activation of aflatoxin B1. There was no detectable effect when the antiserum was added after aflatoxin B1 activation. The result presented suggests that the antibodies in the AFB1-BSA antiserum can interact with aflatoxin B1 prior to its activation. This implies that an immune-protective effect can only be exerted if the antibodies intervene before activation.

Characterization of c-Ki-ras oncogene alleles by direct sequencing of enzymatically amplified DNA from carcinogen-induced tumors

Activated c-Ki-ras genes in liver tumors from rats exposed to the potent hepatocarcinogen aflatoxin B1 were analyzed to determine the nature of their activation by characterization of two c-Ki-ras alleles present in tumor-derived NIH 3T3 mouse transformants. Using selective hybridization of synthetic oligonucleotides to transformant DNA, we have determined that a single G X C to A X T base transition in either the first or second position of the 12th codon is associated with activation of the gene. Such mutations would lead to amino acid substitutions of aspartate or serine for glycine in the mutant proteins. To confirm these findings, we applied a technique for direct sequence analysis of a 90-base-pair region of the rat c-Ki-ras gene produced by primer-directed enzymatic amplification. Findings produced by this approach, which provides a convenient method to characterize mutations in multiple alleles without the necessity to clone individual genes, confirmed the presence and identity of the 12th codon mutations in the activated oncogene, as initially determined by the oligonucleotide hybridization technique.

Mechanism of cytotoxicity of aflatoxin B1: role of cytochrome P1-450

A mouse hepatoma cell line, Hepa-1, is highly sensitive to the toxic effects of Aflatoxin B1 (AFB1). Half maximal survival (LD50) of cells occurs at 0.068 ug AFB1/ml. Benzo(a)anthracene, which induces aryl hydrocarbon hydroxylase and cytochrome P1-450 in Hepa-1, causes a slight increase in the toxicity of AFB1 (LD50 = 0.034 ug/ml). An aryl hydrocarbon hydroxylase- and cytochrome P1-450-deficient mutant of Hepa-1 is, however, over 100 times more resistant to AFB1 than Hepa-1. Almost no decline in survival is observed at 5 ug AFB1/ml. Cytochrome P1-450 thus effects strongly on the cytotoxicity of AFB1 in these cells. The basal activity in Hepa-1 is enough to elicit an almost full toxic effect. AFB1, although a substrate for cytochrome P1-450, does not act as an inducer of aryl hydrocarbon hydroxylase.

Identification of an activated c-Ki-ras oncogene in rat liver tumors induced by aflatoxin B1

Weanling male Fischer rats were administered 40 intraperitoneal injections of aflatoxin B1 (25 micrograms per animal per day) over a 2-month period. This chronic dosing regimen resulted in the sequential formation of hyperplastic foci, preneoplastic nodules, and hepatocellular carcinomas in all of the animals treated. The presence of transforming DNA sequences was detected by formation of anchorage-independent foci after transfection of tumor-derived DNA in NIH 3T3 mouse fibroblasts. Transfection of genomic DNA isolated from individual tumors from eight animals resulted in specific transforming activities ranging from 0.05 to 0.2 foci per micrograms of DNA. Primary transfectant DNAs were analyzed by Southern blot hybridization with DNA probes homologous to c-Ha-ras, c-Ki-ras, and N-ras oncogenes. A highly amplified c-Ki-ras oncogene of rat origin was detected in transformants derived from tumors in two of the eight animals tested. There was no evidence to suggest the presence of c-Ha-ras or N-ras sequences in any of the transformants. Analysis of primary liver tumor DNA showed no Ki-ras DNA amplification when compared to control liver DNA samples. Increased levels of c-Ki-ras p21 proteins were detected in 3T3 transformants containing activated rat c-Ki-ras genes. The presence of c-Ki-ras sequences of rat origin capable of inducing transformed foci can be taken as evidence that the c-Ki-ras gene has been activated in the primary liver tumors.

Effects of dietary Schizandra chinensis, brussels sprouts and Illicium verum extracts on carcinogen metabolism systems in mouse liver

Ethanol extracts of Brussels sprouts (BRX), Schizandra chinensis (SZX) or Illicium verum (IVX) were added to a semi-purified basal diet and fed to adult male and female C57B1/6 mice for 14 and 10 days, respectively. Other groups received the unsupplemented basal diet or a mouse chow. Liver fractions were prepared from these mice to investigate the effects of the diets on the enzyme systems involved in benzo[a]pyrene (BP) and aflatoxin B1 (AFB1) metabolism. The effects of the microsomal or cytosolic liver fractions on the in vitro mutagenicity of BP and AFB1 and on the DNA binding of AFB1 were also studied. There were several apparently sex-related differences in the responses of the monooxygenase system components measured. In males, cytochrome P-450 levels were significantly increased only in the chow group, while significant increases in both 7-ethoxycoumarin O-deethylase (ECD) and aryl hydrocarbon hydroxylase (AHH) activities were seen only in the SZX group. In females, cytochrome P-450 levels were significantly increased in both the BRX and SZX groups, whereas AHH activity was significantly increased only in the chow and BRX groups and ECD activity was increased in the SZX and IVX groups. Microsomal epoxide hydratase (EH) was induced in males in the SZX and IVX groups and in females only in the SZX group, while cytosolic EH was significantly increased only in IVX males. Diet-induced changes in monooxygenase activities were found to be the best indicators of changes in microsome-mediated BP mutagenesis and AFB1 mutagenesis and binding to DNA in vitro, with a direct correlation between high AHH and/or ECD activities and the levels of mutagenic response to BP or AFB1 in the Ames assay and of DNA binding of AFB1.

The in vitro metabolism of aflatoxin B1 catalyzed by hepatic microsomes isolated from control or 3-methylcholanthrene-stimulated rats and quail

The rate of microsomal metabolism of aflatoxin B1 (AFB1) by the male quail hepatic microsomal polysubstrate monooxygenase P-450 in vitro was eight times greater than that catalyzed by male rat cytochrome. In the quail almost all the metabolism proceeded via 8,9-aflatoxin B1 epoxidation (assessed by Tris-AFB1-8,9-dihydrodiol formation), whereas in the rat only 36% of soluble metabolites were by this pathway. Pretreatment with 3-methylcholanthrene in the quail resulted in a 9-fold induction of cytochrome P-450 per unit liver weight, but only a 1.7-fold increase in the rate of aflatoxin B1 metabolism. In the rat the corresponding inductions were 1.7 and 1.3, respectively. The aflatoxin B1 metabolism catalyzed by control and 3-methylcholanthrene-stimulated quail microsomes differed qualitatively from that observed with the corresponding fractions from the rat. It was concluded that not only do the basal aflatoxin B1 microsomal metabolism of rat and quail differ, but also that 3-methylcholanthrene induces cytochromes with very different properties in the two species. Assays of ethoxyresorufin O-deethylase activities and polyacrylamide gel electrophoresis analyses further demonstrated these differences between the species. These studies provide further insight into the metabolic differences underlying individual species sensitivities to aflatoxin B1.

The mercapturic acid pathway metabolites of a glutathione conjugate of aflatoxin B1

A glutathione conjugate of aflatoxin B1 (AFB1) which has previously been identified as 8,9-dihydro-8-(S-glutathionyl)-9-hydroxy aflatoxin B1 (AFB1-GSH) (E.J. Moss, D.J. Judah, M. Przybylski and G.E. Neal, Biochem. J., 210 (1983) 227-233) has been degraded in vitro to all of the intermediates of the mercapturic acid pathway (MAP) and the chromatographic and spectral characteristics of each of these compounds investigated. The cysteinylglycyl conjugate (AFB1-Cys.Gly) was prepared by incubating the AFB1-GSH conjugate with a rat hepatoma cell line rich in gamma-glutamyl-transpeptidase (GGT). Incubations of the AFB1-Cys.Gly conjugate with dipeptidase produced a metabolite, which was purified and characterized by 1H-NMR spectroscopy as 8,9-dihydro-8-(S-cysteinyl)-9-hydroxy aflatoxin B1 (AFB1-Cys). The N-acetyl derivative of the AFB1-Cys conjugate resulted from the incubation of the AFB1-GSH conjugate in vitro with isolated rat kidney cells. Mass spectral data were consistent with the compound being 8,9-dihydro-8-(S-cysteinyl-(N-acetyl))-9-hydroxy aflatoxin B1 (AFB1-Nac.Cys). A chromatographically identical compound was obtained by the chemical acetylation of AFB1-Cys.

Effects of vitamins A, C, and E on aflatoxin B1-induced mutagenesis in Salmonella typhimurium TA-98 and TA-100

The effects of retinoids (vitamin A analogs) and vitamins C and E on the aflatoxin B1 (AFB1)-induced mutagenesis in Salmonella typhimurium TA-98 and TA-100 were investigated. The bioassay was performed under conditions that permitted the effects of vitamins on carcinogen metabolism to be assessed separately from effects on the expression of the mutated bacterial cell. Both retinoic acid and retinol inhibited (up to 50%) AFB1-induced mutagenesis in S. typhimurium TA-98, but only retinol inhibited (up to 75%) mutagenesis in TA-100. Retinoic acid inhibition of mutagenesis in S. typhimurium TA-98 was pronounced over a wide concentration range (i.e., 2 X 10(-10) to 2 X 10(-8) M) however, at the higher concentrations (i.e., 2 X 10(-8) to 2 X 10(-6) M range) the predominant effect was the inhibition of the metabolism of AFB1 to its mutagenic metabolites. Vitamin E was more potent in inhibiting the expression of AFB1-induced mutagenesis than vitamin C. However, the major inhibitory effects of vitamin E were related to the metabolism of AFB1, whereas vitamin C was inhibitory at both metabolic and the post-metabolic levels of the AFB1 mutagenesis assay. The results of these investigations suggest that vitamins A, C, or E inhibit both AFB1 metabolism to its mutagenic metabolites as well as the expression of AFB1-induced mutated bacterial cells.

A critical review of the literature on hydrogen sulfide toxicity

The information available on the biological activity of hydrogen sulfide has been examined for present status of critical results pertaining to the toxicity of hydrogen sulfide. This review of the literature is intended as an evaluative report rather than an annotated bibliography of all the source material examined on hydrogen sulfide. The information was selected as it might relate to potential toxic effects of hydrogen sulfide to man and summarized, noting information gaps that may require further investigation. Several recommendations are listed for possible consideration for either toxicological research or additional short- and long-term tests. Two bibliographies have been provided to assist in locating references considered in this report: (1) literature examined but not cited and (2) reference citations. The majority of the references in the first bibliography were considered peripheral information and less appropriate for inclusion in this report.

Metabolic activation and bacterial mutagenicity of aflatoxin B1 in two different test systems

The metabolism of aflatoxin B1 (AFB1) by liver post-mitochondrial supernatant fraction from phenobarbitone treated male albino rats in liquid suspension and soft-agar plate incorporation bacterial mutation assay systems has been studied. The AFB1 concentration used was in the range of those added to bacterial mutagenicity tests with this mycotoxin. Three oxidative metabolites of AFB1 viz the Tris derivative of AFB1 8,9-diol (derived from the 8,9-epoxide), aflatoxin Q1 (AFQ1) and M1 (AFM1) were observed. The metabolite profile and time course of formation were qualitatively similar in both assay systems. The rate and overall formation of metabolites in the soft-agar system was approximately one half that in the liquid suspension system which was reflected in decreased AFB1 induced bacterial mutation in the former system. The AFB1 metabolite profile in these in vitro systems did not mirror completely the reported in vivo profile seen in the rat, as aflatoxin P1 (AFP1) and a glutathione conjugate were not detected. No evidence was found for a prolongation of metabolism in the soft-agar system.

Restricted repair of aflatoxin B1 induced damage in alpha DNA of monkey cells

We have investigated the processing of adducts formed by covalent binding of aflatoxin B1 (AFB1) to DNA in confluent cultures of African green monkey cells. Repair synthesis elicited by AFB1 adducts was deficient in alpha DNA sequences compared to that in bulk DNA, although the initial levels of modification were the same for these DNAs. The removal of the primary initial adduct, AFB1-N7-Guanine, was deficient in alpha DNA and the kinetics of its loss resembled those previously reported for removal from total DNA in xeroderma pigmentosum cells of complementation group A. Spontaneous loss of the AFB1 moiety or the concomitant loss of the guanine to yield an apurinic site account for these results. The formation of the more chemically stable secondary product, AFB1-triamino-Pyrimidine, occurred more rapidly and to a greater extent in alpha DNA than in bulk DNA, probably because of slower removal of the primary product. The excision repair patch size for AFB1 adducts in alpha DNA was only 10 nucleotides compared to 20 nucleotides for repair of AFB1 adducts in bulk DNA. Irradiation of cells with low doses of UV prior to or immediately after treatment with AFB1 increased the rate and extent of removal of AFB1 adducts from alpha DNA to the levels found in the bulk DNA, indicating that the formation of pyrimidine dimers or their repair may alter the chromatin structure of alpha DNA sufficiently to facilitate its repair.

Activation and deactivation of aflatoxin B1 in isolated rat hepatocytes

Isolated rat hepatocytes took up [3H]-aflatoxin B1 during incubation with fifty percent of the aflatoxin B1 covalantly bound to cellular macromolecules. The amount of bound-aflatoxin B1 was proportional to the medium concentration of aflatoxin B1. The specific radioactivity (pmole/mg) of aflatoxin B1 found in the DNA fraction was 20 fold greater than that associated with protein. Metyrapone (0.75 mM) inhibited significantly the uptake and binding whereas 1,2-epoxy-3,3,3-trichloropropane (0.5 mM) enhanced 2-3 fold both the uptake and binding. Glutathione (0.25 mM) reduced these processes. Results indicate that a transformation of aflatoxin B1 is catalyzed by cytochrome P-450 mixed function oxidase and aflatoxin B1-2,3-epoxide so formed is primarily deactivated by epoxide hydrolase. In the isolated hepatocyte depletion of the epoxide by glutathione apparently has an insignificant role in aflatoxin detoxication.

The effects of excision repair and the plasmid pKM101 on the induction of his+ revertants by chemical agents in Salmonella typhimurium

Expansion of the Ames Salmonella/microsome mutagenesis test to include plasmid pKM101-bearing, excision repair-proficient derivatives permits 1) the identification of mutagens that require both factors for activity; 2) the identification of genotoxins through the enhancement of survival by excision repair; and 3) the classification of substances according to the effects of excision repair on their mutagenesis. Class I includes substances that require excision repair to effect mutagenesis. Class II contains substances whose mutagenesis is not affected by excision repair. Class III mutagens cause premutational lesions in DNA which are readily removed by excision repair. This classification scheme is suggested as a preliminary step in making a risk estimation for a mutagen.

Inactivation of aflatoxin B1 mutagenicity by thiols

The mutagenicity of aflatoxin B1 to Salmonella typhimurium strain TA98 decreased rapidly upon exposure of aflatoxin B1 to various thiols in aqueous solution. Mutagenic activity was reduced to control values within 24 hr with N-acetyl-L-cysteine (NAC), N-2-mercaptopropionylglycine (MPG), mercaptoethanol, reduced glutathione or mercaptopropionic acid at pH values near 4. Mercaptoacetic acid, mercaptosuccinic acid, cysteine, acetyl-D,L-homocysteine thiolactone, cysteine methyl ester, D-penicillamine and beta-mercaptoethylamine were less effective. Relatively high thiol concentrations (greater than or equal to 0 . 25 M) were required to achieve complete inactivation within 24 hr with the thiols tested. The inactivation rate was strongly dependent on thiol concentration and pH, but was relatively independent of the aflatoxin concentration under the conditions examined. With MPG and NAC reaction rates were much slower at neutral pH values than at pH's between 3 and 4. HPLC and thin-layer chromatographic examination of aflatoxin B1 solutions partially inactivated with NAC revealed the formation of a new product at a rate that correlated with the disappearance of aflatoxin B1 and the loss of mutagenic activity. This reaction product has not yet been identified, but the evidence suggests that it is the product of an addition of the thiol at the difuran region of the aflatoxin.

Mutagenicity and cytotoxicity of the carcinogen-mutagen aflatoxin B1 in Streptococcus pneumoniae (Pneumococcus) and Salmonella typhimurium: dependence on DNA repair functions

Strains R6, R6x and R6uvr-1 of Streptococcus pneumoniae (Pneumococcus) are sensitive to the cytotoxic effects of the mutagen/carcinogen aflatoxin B1 (AFB1). R6uvr-1 is more prone to the cytotoxic effects of AFB1 than the repair-proficient parental strain, R6. The same differential susceptibility of strains R6, R6x and R6uvr-1 was observed when UV light replaced metabolically activated AFB1. All pneumococcal strains were immutable by AFB1. AFB1 mutagenesis in Salmonella typhimurium strains was dependent on a functional RecA gene product. The enhancing effects of delta uvrB and plasmid pKM101 were found to be additive. Data presented are consistent with the following: (i) AFB1 toxic effects are due mainly to DNA binding of AFB1; (ii) AFB1 mutagenesis is dependent on error-prone DNA repair; (iii) Pneumococcus lacks an active error-prone (SOS) DNA-repair system.

Epoxides--is there a human health problem?

The purpose of this review is to consider whether epoxides represent a hazard to human health. Possible means of occupational and non-occupational exposure are discussed with reference to the production and uses of industrially important compounds and other epoxides, such as naturally occurring plant and fungal products. In addition to epoxides themselves, unsaturated compounds that may be metabolised in vivo to epoxides are included, since this appears to be a further important means of exposure. The toxicology, in particular carcinogenicity and mutagenicity, is discussed, along with a brief outline of the biochemistry such as metabolism, binding to cell constituents, and DNA repair mechanisms. The question of interactions between different epoxides in vivo is also raised.

The detoxication of aflatoxin B1 with glutathione in the rat

1. The deactivation of aflatoxin B1 by glutathione (GSH) has been investigated in rat. Binding of metabolites of aflatoxin B1 to [3H]glutathione in vitro with rat liver microsomes is insignificant. Incubation with rat liver 10 000 g supernatant results in increased binding. Under identical conditions, benzo(a)pyrene metabolites are bound to [3H]glutathione much more than is aflatoxin B1. 2. Pre-treatment of rats with aflatoxin 1 (2 mg/kg) caused depletion in GSH of rat liver with a minimum at 6 h but returning to above normal at 24 h. GSH S-transferase activity was marginally increased at 6 h also and returned to normal at 24 h. 3. Kidney GSH was not significantly decreased, but kidney GSH S-transferase activity showed a sudden increase in 6 h, returning to almost normal at 24 h. 4. Pre-treatment with benzo(a)pyrene (2 mg/kg) caused greater depletion of hepatic GSH than occurred with aflatoxin B1 but did not show any effect on kidney GSH. 5. Hepatic and kidney GSH S-transferase in benzo(a)pyrene-treated rats showed greatest activity at 2 h followed by a gradual fall through 24 h. 6. GSH was therefore a less efficient nucleophile for aflatoxin B1 metabolites than for benzo(a)pyrene metabolites.

Induction of sister-chromatid exchanges in human lymphocytes and Chinese hamster cells exposed to aflatoxin B1 and N-methyl-N-nitrosourea

The effect of a 1-h exposure to aflatoxin B1 (AFB1) in inducing sister-chromatid exchange in Chinese hamster ovary (CHO) cells and human lymphocytes in the presence or absence of mixed function oxidase ("S9 mix") was compared. CHO cells were also exposed to a graded series of doses of N-methyl-N-nitrosourea, a powerful inducer of SCE whose action was independent of the presence or absence of S9 mix. CHO and human cells showed a close correlation in response to SCE induction by AFB1 and in both cell systems the additon of mixed function oxidases in the S9 mix resulted in a marked enhancement of action of AFB1.

Conditions for induction of bacteriophage from lysogenic Bacillus megaterium with aflatoxin B1

The present study was conducted to determine whether or not aflatoxin B1 was an effective inducing agent for lysogenic bacteria and to characterize some of the parameters involved in induction. A lysogenic strain of Bacillus megaterium (NRRL-B-3695) and an indicator strain of this species (NRRL-B-3694) were used. Cultures of the lysogenic strain were incubated for various periods of time in the presence of aflatoxin B1. Plaque-forming units as well as colony-forming units were then determined. Results of the present study indicated that bacteriophage lysogenizing B. megaterium could be induced with aflatoxin B1. The optimum concentration for induction was 25 micrograms of toxin per ml of early-log-phase culture. Evidence suggested that: (i) higher concentrations of aflatoxin B1 formed hydrophobic complexes which would not efficiently induce B. megaterium; (ii) the toxic effect of aflatoxin B1 severely limited the number of cells which could be induced prior to killing action of the toxin; and (iii) concentrations less than 25 micrograms of aflatoxin B1 per ml were not efficient inducers of bacteriophage production nor did they demonstrate the toxic effect observed at higher concentrations.

Some high-performance liquid-chromatographic studies of the metabolism of aflatoxins by rat liver microsomal preparations

The metabolism of aflatoxin B1 in vitro was examined in rat liver microsomal preparations. 2. H.p.l.c. (high-performance liquid-chromatographic) systems were used. A silica column was used to separate non-polar metabolites. A system utilizing a reversed-phase column which separates both poar and non-polar metabolites was also developed. 3. The principal metabolites of aflatoxin B1 found were aflatoxin M1, aflatoxin Q1 and a compound which co-chromatographed with a degradation product of aflatoxin B1 2,3-dihydrodiol. 4. The time course of metabolism of aflatoxin B1 by microsomal preparations isolated from control and phenobarbitone-pretreated rats was examined. The rate and extent of metabolism was greater with microsomal preparations from the latter. The formation of aflatoxin Q1 was enhanced 4--5-fold by phenobarbitone pretreatment, whereas the production of aflatoxin M1 was only increased 1--2-fold. The formation of the degradation product of aflatoxin B1 2,3-dihydrodiol was increased 4--5-fold by the pretreatment with phenobarbitone. 5. The microsomal metabolism of aflatoxins M1, P1 and Q1 was examined. Aflatoxin M1 apparently underwent very limited microsomal metabolism to more polar compounds. Aflatoxin P1 was not metabolized. The situation with aflatoxin Q1 was complicated in that it was metabolized in the absence of NADPH to an unidentified metabolite. Aflatoxin B1 appeared as a metabolite of aflatoxin Q1 only when NADPH was present, and the formation of more polar metabolites was also then observed.

The major metabolite of aflatoxin B1 in the rat is a glutathione conjugate

[14C]aflatoxin B1 (AFB1) was injected i.p. into female Wistar rats. Half of the dose was eliminated into the bile mostly as polar non-extractable metabolites. Among these a glutathione conjugate was the main component. The same conjugate was formed when rat liver postmitochondrial supernatant was incubated with AFB1 and [3H]glutathione. The conjugate was purified by ion exchange chromatography, gel-filtration and thin layer chromatography (TLC). It was tentatively identified as 2,3-dihydro-2-(S-glutathionyl)-3-hydroxy aflatoxin B1 (AFB1-GSH-conjugate). This structure was derived mainly from amino acid analysis, ultraviolet spectra and the enzymatic requirements for its formation in in vitro experiments. In the rat this detoxification product of the potentially ultimate reactive AFB1-epoxide constitutes about 10% of the administered dose and thus underlines the quantitative importance of this activating pathway.

Modification of DNA by aflatoxin B1 creates alkali-labile lesions in DNA at positions of guanine and adenine

The damage to DNA by the hepatocarcinogen aflatoxin B1 was investigated. A DNA fragment of known sequence of the lactose promoter-operator region was used as a substrate for modification by aflatoxin B1. The DNA was incubated with aflatoxin B1 in crude mammalian liver extracts or with purified microsomes. Treatment of the DNA incubated in the complete system with either 1 M piperidine or 0.1 M NaOH at 90 degrees revealed alkali-labile lesions in the DNA. The exact location of the cleavage site was determined by comparison of the length of the cleavage products with the known sequence on polyacrylamide gels. The lengths of the cleavage products were the same as those produced by alkali-induced breakage of the same sequence of DNA that had been modified with dimethyl sulfate. The major cleavage products of the aflatoxin B1-modified DNA were at positions of guanine and the minor cleavage products were at positions of adenine. These studies show that modification of DNA by aflatoxin B1 creates alkali-labile sites at positions of guanine and, to a lesser extent, adenine.

Identification of the principal aflatoxin B1-DNA adduct formed in vivo in rat liver

The products of in vivo covalent binding of activated aflatoxin B1 (AFB1) to DNA have been investigated in rats. The principal covalent product formed in liver DNA of rats treated with AFB1 has been identified as 2,3-dihydro-2-(N7-guanyl)-3-hydroxy-aflatoxin B1. This compound was isolated from the liver DNA of rats dosed with AFB1 (2.0 mg/kg) in sufficient quantity for characterization by physicochemical techniques, including field-desorption mass spectrometry. This information together with results of chemical methylation of the compound proved that the major adduct formed between DNA and AFB1 in vivo is identical to that produced in vitro when AFB1 is incubated with DNA in the presence of a rat liver microsomal activating system. Quantitative studies of formation of this compound revealed a dose-dependent relationship between the level of its occurence in liver DNA and AFB1 doses over the range 0.125-1.0 mg/kg.

Expression of a bacterial gene turned on by a potent carcinogen

Contrary to mutagenesis, lysogenic induction produced by chemical carcinogens occurs in the majority of a population of lysogenic cells. Such a mass effect can therefore be measured at the biochemical level using an E. coli tester strain in which the galactose operon has been put under the negative control of the lambda repressor. In this publication we show that galactokinase synthesis is turned on by aflatoxin B1 metabolites within an hour after treatment of the tester bacteria. Such a biochemical assay provides a useful means for identifying potential chemical carcinogens.

Effects of the epoxide hydrase inhibitor, 1,1,1-trichloropropane-2,3-oxide on the genetic activity of aflatoxin B1 metabolites in in vitro activation test systems

The epoxide hydrase inhibitor 1,1,1-trichloroprophane-2,3-oxide (TCPO) was genetically active to cells of S. cerevisiae and conidia of N. crassa. This genetic activity could be eliminated or reduced to near spontaneous levels in the presence of the S-9 fraction of hamster liver homogenate. The addition of TCPO to an in vitro activation system containing aflatoxin B1 resulted in an increase in the genetic activity of aflatoxin B1, and this increase was dependent on the dose of TCPO. These results are discussed in relation to the possible metabolism of the promutagen aflatoxin B1.